1. Field of the invention
The invention relates to the production of aluminium by means of igneous electrolysis, particularly using the Hall-Heroult electrolysis process, and installations intended for the industrial embodiment of said production. The invention relates more specifically to the control of thermal flows from electrolytic cells and the cooling means used to obtain this control.
2. State of the related art
Metal aluminium is produced industrially by means of igneous electrolysis, i.e. by electrolysis of alumina in solution in a molten cryolite-based bath, referred to as an electrolyte bath, particularly according to the well-known Hall-Heroult process. The electrolyte bath is contained in pots, referred to as “electrolytic pots”, comprising a steel shell, the inside of which is lined with refractory and/or insulating materials, and a cathode assembly located at the base of the pot. Anodes are partially immersed in the electrolyte bath. The expression “electrolytic cell” normally refers to the assembly comprising an electrolytic pot and one or more anodes.
The electrolysis current circulating in the electrolyte bath and the liquid aluminium pad via the anodes and the cathode components and which may reach intensities greater than 500 kA, carries out alumina reduction reactions and also makes it possible to maintain the electrolyte bath at a temperature of the order of 950° C. by means of the Joule effect. The electrolytic cell is fed regularly with alumina so as to compensate for the alumina consumption resulting from the electrolysis reactions.
The electrolytic cell is generally controlled such that it is in thermal equilibrium, i.e. the heat dissipated by the electrolytic cell is compensated overall by the heat produced in the cell, which essentially comes from the electrolysis current. The thermal equilibrium point is generally selected so as to achieve the most favourable operating conditions in not only technical, but economic terms. In particular, the possibility to maintain an optimal set-point temperature represents an appreciable saving on the production cost of aluminium due to the maintenance of the current efficiency (or Faraday yield) at a very high value, reaching values greater than 95% in the most efficient plants.
The thermal equilibrium conditions depend on the physical parameters of the cell (such as the dimensions and nature of the constituent materials or the electrical resistance of the cell) and the cell operating conditions (such as the bath temperature or the electrolysis current). The cell is frequently constituted and run so as to induce the formation of a ridge of solidified bath on the lateral walls of the pot, which particularly makes it possible to inhibit corrosion of the linings of said walls by the liquid cryolite.
In order to be able to achieve very high electrolysis current values in restricted electrolytic pot volumes, it is known to equip the electrolytic cells with specific means to evacuate and dissipate, possibly in a controlled manner, the heat produced by the electrolytic cells.
In particular, in order to favour solidified bath ridge formation more specifically, it is known, through the American patent U.S. Pat. No. 4,087,345, to use a shell equipped with stiffeners and a reinforcement frame constituted so as to favour the cooling of the sides of the pot by natural convection of ambient air. These static devices do not lend themselves easily to precise thermal flow control.
It has also been proposed, through the patent application EP 0 047 227, to reinforce the heat insulation of the pot and equip it with heat pipes equipped with heat exchangers. The heat pipes pass through the shell and the heat insulator and are incorporated in the carbonaceous parts, such as the edge slabs. This solution is relatively complex and costly to implement and also results in significant modifications of the pot.
The French patent application FR 2 777 574 (corresponding to the American patent U.S. Pat. No. 6,251,237), held by Aluminium Pechiney, discloses an electrolytic cell cooling device using air blowing with localised jets distributed around the shell. However, the very high efficiency of this device is limited by the intrinsic heat capacity of the heat transfer fluid.
Having noted the absence of sufficiently satisfactory known solutions, the applicant set an objective to find effective and adaptable means to evacuate and dissipate the heat produced by the electrolytic cell, which can easily be implemented and does not require significant modification of the cell, particularly of the shell, a large infrastructure, or redhibitory additional operating costs. With a view to use the same in both existing plants and new plants, the applicant particularly researched means which make it possible to modify the power of the cells, which can be adapted easily to various cell types or at different operating modes of the same cell type, and which lend themselves to industrial installations comprising a large number of cells in series.